One commonly used laser-machining (laser-processing) method involves modulating a continuous wave (CW) or pulsed laser-beam using an AOM. Radiation used for the machining is admitted to a workpiece via the AOM at one angle of incidence thereon for effecting the machining, and directed away from the workpiece via the AOM at another angle of incidence thereon during a pause in the machining.
Traditionally, the beam is admitted to the workpiece by the AOM by diffracting the beam at a first-order diffraction angle (direction) of the AOM, and directed away from the workpiece by transmitting the laser-beam through the AOM at a zero-order incidence angle (direction) of the AOM. More recently, however, it has been found advantageous to use the zero-order transmission of the AOM to admit the laser-beam to the workpiece, and the first-order diffraction to direct the laser-beam away from the workpiece.
This latter method is preferred for laser-beams having a relatively broad spectral content, such as beams from carbon monoxide (CO) lasers, as no dispersion of the laser-beam occurs on zero-order transmission. This avoids a need to provide means to correct dispersion before delivering the laser-beam to the workpiece for the machining or processing. A method in which machining is effected by a first order AOM-diffracted beam is described in U.S. Pre-grant Publication No. 20150083698, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference. A method in which machining is effected by a zero-order AOM-transmitted beam is described in U.S. Pre-grant Publication No. 20170050266, assigned to the assignee of the present invention, and the complete disclosure of which is also hereby incorporated herein by reference.
Laser-beams used in AOM-modulated laser processing methods are typically plane-polarized, and diffraction by an AOM is polarization sensitive. It has been found in cases where diffraction by the AOM is used to direct a laser-beam away from a workpiece that there is some “leakage” of laser-radiation along the zero-order transmission direction. This has been found to be as much as about 2% of the incident laser-radiation. The leakage can be due to deviation from exact plane-polarization of the laser-beam or by slight misalignment of the polarization-plane with the AOM.
In many applications, such a leakage may be below a threshold value at which a workpiece could be altered or damaged in some way and can accordingly be ignored. In some sensitive applications, however, or in an application where the leakage may strike repeatedly on a workpiece at the same spot, the threshold could be exceeded with negative consequences. There is a need to reduce such leakage, preferably by about an order of magnitude.